Conventional burners, in widespread commercial use today, whether of the residential or commercial type, continuously combust air (oxygen) and fuel. Such burners will be referred to in this specification throughout as "continuous burners". In all such burners, combustion air (or oxygen) and fuel are metered at precise rates into a burner body where the fuel and combustion air is mixed into a combustible mixture and ignited. The combustion is stabilized and a continuous flame is propagated from the stabilization point, the air and fuel being combusted in the flame front. Such conventional burners are consistent and reliable and they are generally quiet. Further, their design, even for highly fuel efficient designs, has developed into widely accepted design principles which are universally followed to yield commercially dependable burners.
Developments in continuous burners have also led to improvements in their turndown ratio. Because turndown ratio can be expressed in different ways, as used herein, "turndown ratio" means the ability of the burner to vary its total heat output over a fixed period of time. In this area development work continues since it is desirable to produce a burner which can maintain stoichiometric to "lean" combustion over a wide turndown radio. In conventional continuous burner design, turndown is accomplished by varying the rate at which combustion air and fuel are fed into the burner, but not the ratio therebetween which is fixed. Depending on the burner design there is an upper and lower mass flow rate at which combustion can no longer be regularly sustained and this determined the turndown ratio for any particular burner. Another turndown approach which has gained commercial acceptance is referred to as pulsed combustion which will be described below. In pulsed combustion, the fuel and air to the burner are periodically regulated to be on-off in variable cycles (usually controlled by microprocessors) and in this manner the total heat output over a given period of time can be regulated. Continuous burners have typical turndown ratios of 3:1 to 6:1 and in some instances have gone as high as 10:1.
In spite of their widespread use, continuous burners have limitations. The turndown ratio, even in pulsed combustion, is limited. Complete combustion is always a problem and even with so-called stoichiometric continuous burners, certain pollutants such as nitrous oxide emissions exist at a level higher than that which would theoretically exist if the combustion were instantaneous for a fixed volume of fuel and air. Inherently, both the gas and air supplied to the burner must be pressurized. Also, a conventional, continuous burner is capable of only heating the work or the environment, although in some heat treat applications the combustion air in the burner may be used to cool the work if the fuel is turned off.
An alternative to continuous combustion is a process known as pulse combustion. Pulse combustion is an old technology. One of the best known examples of a pulse combustor is the German V-1 "Buzz Bomb" used in World War II. A more recent example of a pulse combustor is the recently developed Lennox space heater which is operated as an acoustic Helmholtz resonator. The pulse combustion principle is illustrated in FIGS. 1A-1D.
In FIG. 1A, the start-up of the cycle is illustrated. Combustion air 1 and fuel 2 are introduced simultaneously through a pair of flapper valves which function as one-way pressure sensitive check valves. These reactants are mixed in the combustion chamber and initially ignited by a spark plug 5. A rapid combustion (FIG. 1B) results which produces a pressure surge that advances upstream to slam shut the inlet valves and block off the entrance preventing further fuel and combustion air from entering the combustion chamber. At the same time, a pressure pulse 6 travels downstream to produce a surge of the products of combustion out of the exhaust duct as shown in FIG. 1B. When the products of combustion are discharged from the combustion chamber, the pressure in the chamber tends to drop. Inertia causes the products of combustion in the exhaust duct to continue to flow through the discharge duct even after the explosion pressure in the combustion chamber has been dissipated. Conventional, accepted thinking is that the wave motion or pulse of the products of combustion drops the pressure in the combustion chamber below atmosphere with the result that the inlet flapper valves open causing a further mixture of air and fuel to enter the chamber as shown in FIG. 1C. The cycle is then repeated. It is also known that the mixture in FIG. 1C can be ignited from the hot gas residue of the previous cycle causing the process to be self-sustaining. The process is usually driven acoustically typically at the resonance frequency.
There are several different pulse combustor designs which all operate on the same underlying principle, i.e. the periodic addition of fuel and air must be in phase with the periodic pressure oscillations. In the literature, the pulse combustors are generally identified as the quarter wave or Schmidt tube, the Rijke tube and the Helmholtz resonator. Referring to FIG. 1A, the Lennox space heater operates as an acoustic Helmholtz resonator with its small neck replaced by a tailpipe. The German V-1 "Buzz Bomb" operated as a quarter wave tube in that the tailpipe as shown in FIG. 1A was shaped as an exhaust duct with combustion occurring at a distance x=length/4 which generated a thrust harnessed for propulsion. The Rijke tube is similar to the quarter wave or Schmidt tube and comprises a vertical tube open at both ends which contains a heat source in the center of its lower half, that is at x=length/4. The Rijke combustor is generally used with liquid fuel because the upward flow of heat from the heat source can be utilized to volatilize the fuel to produce the combustion at the desired location. There have been countless design variations. Generally, combustion air may be premixed with the fuel and/or fuel premixed with the air and/or a premixing chamber utilized in conjunction with the combustion chamber. Principally, gaseous fuel can be 1) premixed with entering air; 2)fed continuously to the combustion chamber; 3) supplied from a plenum through a separate aerodynamic valve; or 4) supplied from a tuned chamber. In all pulse combustors, the fuel and air quantities are mixed and then brought, more or less as a total mixture, into an explosive ignition which produces the noise associated with the devices, and generates the pulsed pressure waves which control the fuel and air combustion. Typically, flapper valves as shown in FIGS. 1a-1c simultaneously admit and mix the fuel and air as they are drawn into the combustion chamber. In the tube arrangements discussed, the air may be drawn into the tube vis-a-vis a flapper valve while the fuel is emitted downstream in the tube. The fuel and air mix as they travel further downstream to the point where the total mixture is explosively ignited and this ignition/combustion produces the noise and shock typically associated with pulse combustion.
As thus defined, pulse combustors are generally recognized to have certain advantages over the steady state combustion employed in continuous burners used in most boilers and furnaces. The advantages include:
a) Because of the sudden combustion, pulse combustors are believed to have combustion intensities that are up to an order to a magnitude higher than conventional burners. PA1 b) Pulse combustors are generally believed to have heat transfer rates that are a factor of two to three times higher than continuous burners. This results because in most pulse combustors, the combustion occurs near the closed end of a tube where inlet valves operate in phase with pressure amplitude variations to produce localized temperature and pressure oscillations around a means value. More specifically, it is known that flow oscillations can significantly increase heat transfer over a steady turbulent flow and the oscillations, if large enough, can in themselves create additional turbulence increasing heat transfer. This means that more heat can be removed with a smaller more compact heat exchanger thus decreasing the overall cost of a furnace or heater. PA1 c) Because of the suddenness of the combustion, it is generally believed that nitrous oxide emissions are reduced or lowered by as high a factor as three. PA1 d) Finally, pulse combustors are inherently self-aspirating since the combustor generates a pressure boost. This obviates the need for a blower and also permits the use of a compact heat exchanger that may include a condensing section which obviates the need for chimney or a draft, an important consideration in many applications. PA1 i) All pulse combustion systems produce objectionable noise whether the systems are acoustically driven or otherwise. This is inherent because the combustible mixture is formed from the complete charge which produces an explosive ignition. A typical approach which is followed to mute the noise is a system using pairs of pulsed combustors which must be operated in phase at or near resonance so that the pressure or noise from one unit cancels the noise or pressure pulse of the other. The pressure or noise is not eliminated and along with the noise is shock resulting from the explosion. The chamber and tailpipe have to be designed to withstand the shock. PA1 ii) The second principal defect present in current pulse combustors is the fact that they posses little if any turndown ratios. For example, acoustically driven pulse combustors operate at one combustion speed, the resonance frequency. As noted above, all pulse combustors are operated in self-sustaining phase such that the fuel and air is admitted in periodic phase relationship with the pressure oscillations resulting from the explosion of the air and fuel. This means that the entire arrangement has to simply be operated on/off to achieve turndown. Any attempt to achieve turndown by varying the charge of fuel plays havoc with the interaction of combustion chamber geometry and combustion oscillations which are precisely configured to insure sudden combustion at a fixed volume of fuel and combustion air. PA1 iii) Finally, and notwithstanding the commercial success of certain prior art pulse combustion systems such as the Lennox system, there is in general a reliability or consistency problem affecting prior art pulse combustion systems. As noted, the success of any pulse combustion system is critically dependent on the geometry of the combustion cavity and this geometry is presently determined by trial and error to produce a specific combustor geometry for a specific application which is characterized by a narrow turndown ratio and some form of attachment to mute the noise resulting from ignition explosion. For example, besides considerations relating to combustion chamber geometry, the tailpipe is sized relative to the exhaust opening to create a back pressure. If the exhaust opening is very small, such as approaching that of an orifice, the noise resulting from the combustion explosion increases, etc. Thus, the tailpipe back pressure plus exhaust opening must be considered in the design, etc. The design parameters which permit consistently reliable pulse combustion burners to be built have not been developed. PA1 1) The fuel inlet is arranged as a manifold with a plurality of jets spaced radially outwardly from the longitudinal center of the chamber and directed towards the stabilizing rod to insure or improve the "soft" ignition or muted "explosion" characteristics of the device while enabling the device to be built as a compact unit with a minimal combustion chamber length and at the same time maintaining or improving the thorough combustion characteristics of the burner. In accordance with a more specific aspect of this feature of the invention, the design is optimized by employing a plurality of relatively small, high speed jet streams which are angularly orientated to intersect the longitudinal centerline of the combustion chamber adjacent the stabilizing rod and spark electrode. PA1 2) An elastic one-piece reed valve formed of silicon rubber having a plurality of circular appendages extending from a circular hub abuts against the combustion chamber's axial end plate which has circular air inlet openings such that the appendages can instantaneously seal the openings when back pressure is created during combustion while quickly admitting combustion air into the combustion chamber upon completion of the combustion cycle step. The configuration of the valve in combination with its composition provides a simple and effective sealing arrangement which is extremely fast in its response time and materially enhances the efficiency of the device from an overall cycle point of view. PA1 3) In combination with the feature of Paragraph 2, an orifice is placed in the exhaust opening or the exhaust opening of the chamber is significantly reduced in size and the one-way valve in the parent invention is eliminated. The exhaust orifice reduces the exhaust opening of the combustion chamber to a small area when compared to the inlet area opening to insure that combustion air and not flue products will fill the chamber during the suction stroke of the cycle while, because of its size, permitting the creation of a large back pressure within the chamber during the combustion stroke, thus enhancing the overall operation of the cycle while simplifying the arrangement. At the same time, the driven pulsation characteristics of the invention coupled with the unique combustion over a timed interval permits a small orifice opening to be used to generate a forceful pressure pulse with minimal noise. PA1 4) In accordance with improved heat transfer aspect of the invention, an annular or torroidal manifold is provided and at least two L-shaped tubes within the container are connected with short leg portions to the exhaust outlet of the combustion chamber and long leg portions to the manifold. At least two second longer L-shaped tubes or coils are connected to the arrangement such that the longer length leg portions are connected to the annular manifold and the short leg portions are connected to the outlet of the container. When the pulse combustion type burner is operated, water vapor is inherently produced as a product of combustion and upon initial application of the burner the water vapor will condense to water potentially blocking the exhaust until such time as the heat from the burner about the exhaust exceeds the dew point (130.degree. F.). Accordingly, by providing at least one and preferably a plurality of short and long L-shaped tubes, a reservoir resides in the lower legs and bottom portion of the manifold to collect the condensed water vapor while permitting the remaining flue gases to be freely exhausted through the other leg. PA1 5) In accordance with another system aspect of the invention, the pulse line is tapped by a pressure sensing device which in turn is connected either directly or indirectly through the valve timing arrangement to the fuel shut-off whereby the burner flame can be monitored or supervised to permit automatic shut-off of the burner in compliance with safety regulations without the need for using more complicated standard flame supervision systems. PA1 a) self-aspiration of combustion air obviating need for combustion air blower, PA1 b) reduced NO.sub.x emissions, PA1 c) higher heat transfer performance, PA1 d) fuel savings, and PA1 e) chimney or draft device elimination.
While the advantages of pulse combustion when compared to conventional steady state combustion devices are significant, there are serious disadvantages associated with pulse combustion which has heretofore prevented their wide scale commercial acceptance. The disadvantages include:
Within the patent publication art, UK Patent 1,040,478 discloses a pulsating type combustion apparatus which at first glance, appears to be similar to the present invention in that reed valves are employed in the device and the fuel admittance can be optionally timed. However, the pulse cycle is controlled so that a fuel-air combustible mixture is introduced into the combustion chamber and ignited in the same manner as that of the pulse combustors described above. An explosive noise is then generated when the explosive mixture is ignited. Again, it is known to pulsate the fuel supply to a pulse combustion device, such as illustrated in UK patent 1,432,344 l(as well as UK patent 1,040,478), but a combustible mixture is initially introduced into the combustion chamber where it is exploded to produce the pulse from which the process is named.